In collaboration with their German and Czech colleagues, the researchers from the National Research Nuclear University MEPhI Institute for Laser and Plasma Technologies have developed a new generation method of superstrong quasistatic electrical fields accelerating ions in laser plasma. The research results are of great importance for medicine, and in particular for proton therapy, the modern cancer treatment approach. The paper is published in the prestigious journal Scientific Reports.

There are three major approaches to cancer treatment: surgery, chemotherapy and radiation therapy. The latter requires treating tumor with ionizing radiation that affects badly both the tumor itself and the surrounding tissue. There is thus limitation on power of gamma-rays used in radiation therapy.

That is why it is much better to use protons. Due to their relatively large mass, they do not scatter across the tissue or penetrate it too deep allowing scientists to precisely focus the beam on a tumor without damaging the surrounding healthy tissue.

However, a charged particle accelerator needed for producing proton beams is a multi-ton and costly facility. A syncrocyclotron of the Orsay Protontherapy Center (France), for instance, weighs a total of 900 tons. That is why many institutes around the globe are currently engaged in research on alternate methods of ultrafast charged particles beams generation with one of them being based laser accelerator.

Laser charged particle beams accelerators are much more compact and cheaper than common cyclotrons and synchrotrons, though their quality so far has remained insufficient for most applications due to wide range of energy levels and low power of protons. A new laser acceleration method race has now taken place as obtaining a proton beam of 100-200 MeV with energy range of few per cents would usher in a new are in laser medicine.

Pursuant to the MEPhI scientists, the theory they developed may contribute to developing new laser acceleration methods. ”In this research, we theoretically predicated and calculated an effect that seems to be rather paradoxical: the way the radiation reaction force affects charged particles emitting electromagnetic waves may contribute to their acceleration”, told Evgeniy Gelfer, associate professor of the MEPhI Theoretical Nuclear Physics Department and research scientist of Extreme Light Infrastructure Beamlines Institute (Czech Republic).

In ordinary mechanical systems, reaction forces always lead to the loss of kinetic energy and attenuation of ordered motion. The radiation reaction force, however, is a whole different story as it appears due to the transfer of external field energy (laser one in this case) into that of very high-frequency quanta. The transfer is carried out by an electron able to slow down or speed up during the process.

“We studied the ultrastrong laser impulse propagation in plasma. In electromagnetic fields of several petawatt and more (1 PW equals to one quadrillion watt; by comparison, the capacity of the biggest power plant is 22,500 MW, i.e. roughly 50,000 times less), the electron radiation is so intense that their movement is defined not only by the Lorentz force but also by the radiation reaction force occurring as a result of radiation recoil. The latter can even exceed the Lorentz force level. We showed that slowing down the electrons by the radiation reaction force in a plane perpendicular to the laser beam propagation, results in accelerating them forward. Therefore it contributes to more effective charge separation in plasma and stronger longitudinal electrical field occurring as the result of it. That is this field that induces electron accelerating, and that’s why our research results can help to obtain the beams of higher quality”, says Evgeniy Gelfer.